Recent Articles

Verifying copies

Published by Jim Salter // June 29th, 2016

Most of the time, we explicitly trust that our tools are doing what we ask them to do. For example, if we rsync -a /source /target, we trust that the contents of /target will exactly match the contents of /source. That’s the whole point, right? And rsync is a tool with a long and impeccable lineage. So we trust it. And, really, we should. But once in a while, we might get paranoid. And when […]

Read More

PSA: snapshots are better than ZVOLs.

Published by Jim Salter // June 16th, 2016

A lot of people new to ZFS, and even a lot of people not-so-new to ZFS, like to wax ecstatic about ZVOLs. But they never seem to mention the very real pitfalls ZVOLs present. What’s a ZVOL? Well, if you know what LVM is, a ZVOL is like an LV, but for ZFS. If you don’t know what LVM is, you can think of a ZVOL as, basically, a dynamically allocated “raw partition” inside ZFS. […]

Read More

ZFS: practicing failures on virtual hardware

Published by Jim Salter // May 16th, 2016

I always used to sweat, and sweat bullets, when it came time to replace a failed disk in ZFS. It happened infrequently enough that I never did remember the syntax quite right in between issues, and the last thing you want to do with production hardware and data is fumble around in a cold sweat – you want to know what the correct syntax for everything is ahead of time, and be practiced and confident. […]

Read More

testing the resiliency of zfs set copies=n

Published by Jim Salter // May 9th, 2016

I decided to see how well ZFS copies=n would stand up to on-disk corruption today. Spoiler alert: not great. First step, I created a 1GB virtual disk, made a zpool out of it with 8K blocks, and set copies=2. me@locutus:~$ sudo qemu-img create -f qcow2 /data/test/copies/0.qcow2 1G me@locutus:~$ sudo qemu-nbd -c /dev/nbd0 /data/test/copies/0.qcow2 1G me@locutus:~$ sudo zpool create -oashift=12 test /data/test/copies/0.qcow2 me@locutus:~$ sudo zfs set copies=2 test Now, I wrote 400 1MB files to it […]

Read More

zfs: copies=n is not a substitute for device redundancy!

Published by Jim Salter // May 2nd, 2016

I’ve been seeing a lot of misinformation flying around the web lately about the zfs dataset-level feature copies=n. To be clear, dangerous misinformation. So dangerous, I’m going to go ahead and give you the punchline in the title of this post and in its first paragraph: copies=n does not give you device fault tolerance! Why does copies=n actually exist then? Well, it’s a sort of (extremely) poor cousin that helps give you a better chance […]

Read More
  • "I honestly feel as though my business would not be where it is today were it not for us happening into the hiring of Jim Salter."

    W. Chris Clark, CPA // President // Clark Eustace Wagner, PA

  • "Jim’s advice has always been spot on – neither suggesting too little or too much. He understands that companies have limited resources and he does not offer up solutions beyond those that we have truly needed."

    Paul Yoo // President // US Patriot Tactical

  • "Jim Salter is an indispensable part of our team, maintaining our network and being proactive on all of our IT needs. We have comfort knowing that there are redundant bootable backups of all files and databases, offsite and onsite."

    Regina R. Floyd, AIA, LEED AP BD+C // Principal // Watson Tate Savory

  • Recent Thoughts

  • Verifying copies
  • Most of the time, we explicitly trust that our tools are doing what we ask them to do. For example, if we rsync -a /source /target, we trust that the contents of /target will exactly match the contents of /source. That’s the whole point, right? And rsync is a tool with a long and impeccable lineage. So we trust it. And, really, we should. But once in a while, we might get paranoid. And when […]